A variety of applications require the use a uniform line image formed from a high-power laser beam. One such application is laser thermal processing (LTP), also referred to in the art as laser spike annealing (LSA) or just “laser annealing,” which is used in semiconductor manufacturing to activate dopants in select regions of a semiconductor wafer when forming active microcircuit devices such as transistors.
One type of laser annealing uses a scanned line image formed from a laser beam to heat the surface of the wafer to a temperature (the “annealing temperature”) for a time long enough to activate the dopants but short enough to minimizing dopant diffusion. The time that the wafer surface is at the annealing temperature is determined by the power density of the line image, as well as by the line-image width divided by the velocity at which the line image is scanned (the “scan velocity”).
One type of high-power laser that is used for laser annealing applications is CO2 laser. Traditional methods of performing laser annealing with a CO2 laser including imaging the light beam onto a pair of knife-edges and then relaying the light passing therethrough to an image plane to form the line image. The knife-edges are positioned to transmit only a narrow central portion (e.g., 10%) of a Gaussian laser beam for which the intensity is relatively uniform so that the resulting line image is also relatively uniform along the length of the line image.
Unfortunately, using only the narrow central portion of the laser beam means that the other 90% of the light beam is rejected. This is a very inefficient use of the high-intensity laser light. On the other hand, the conventional wisdom is that trying to pass a larger portion of the Gaussian beam will naturally result in non-uniformity of the line image along its length because of the substantial drop off in intensity in the Gaussian beam with distance from the center of the beam.
Furthermore, there are applications where it is advantageous to perform a defect anneal and a spike anneal simultaneously. In this regard, the CO2 laser beam is combined with a broader laser beam, typically from a diode laser. The broader laser beam raises the temperature of the surrounding area to an intermediate temperature for a longer period of time than the CO2 beam, which is used to “spike” the surface to about 1300° C. for a millisecond or less. Typically, the broader laser beam will heat the region for several milliseconds (e.g., in the range from 2 milliseconds to 20 milliseconds) to an intermediate temperature between 700 and 1200° C. The total power required by the diode laser to heat the substrate to this temperature and temporal range is large, e.g., typically several killowatts (kW). Integrating these two laser beam is typically challenging. In a conventional system, the CO2 laser beam and the diode laser beam are not collinear because the optics required to deliver the beams to the wafer are significantly different.
In addition, an important constraint in the design of the laser annealing tool is the avoidance of the incoming laser beam onto the sidewall of the wafer. The laser beams are incident to the surface of the wafer at Brewster's Angle, which is about 70° for silicon. At this incident angle, the power density on the side of the wafer is greater than three times the power density on the wafer surface, and can damage, or even break, the wafer. It has been shown in U.S. Pat. No. 8,071,908 that a serrated skirt can protect the sidewall of the wafer with an incident CO2 laser beam. However, the additional (diode) laser also needs to avoid the sidewall of the wafer because the diode laser provides a large amount of power, e.g., 3 kW typically. It turns out that, geometrically, it is an over constrained problem to design a skirt to protect the wafer from a CO2 laser incident from one direction, and a diode laser incident at nominally 90 degrees from the CO2 laser beam. Hence, it becomes impracticable to use a diode laser with such high power without taking costly and/or time-consuming steps to avoid damaging or breaking wafers.
A further disadvantage of the above approach comes from “pattern effects”. Pattern effects are temperature non-uniformities that arise due to patterns on the wafer. The patterns are features of the devices and interconnections being formed. The pattern effects are much more significant when the incident laser has a shorter wavelength (i.e., closer to visible wavelengths of light) because the pattern effects are driven by Raleigh scattering, which scales as the ratio of feature or pattern size δ divided by the wavelength λ, raised to the fourth power, e.g., (δ/λ)4.